The present invention generally relates to the field of digital radio-frequency or intermediate-frequency receivers, and more particularly to a tunable sigma-delta modulator for utilization in a digital receiver.
Very large scale integration (VLSI) technology is typically utilized to achieve significant reductions in the size, power dissipation and cost of electronic circuits. However, current digital radio receiver architectures prevent the efficient application of very large scale integration because the large amount of analog signal processing needed in the frequency synthesizers and the analog frequency translators is simply not amenable to VLSI applications. For example, high performance analog frequency synthesizers require large, shielded voltage controlled oscillators (VCOs), intermediate frequency (IF) filters typically utilize discrete crystal, ceramic, surface acoustic wave (SAW) or mechanical components, balanced mixers often require transformer coupling, and automatic gain control (AGC) attenuators often utilize p-i-n diodes in their design.
One way in which very large scale implementation of a digital radio may be implemented is by replacing the functions of the analog frequency synthesizers and analog frequency translators with high speed, wide bandwidth active filters which are amenable to VLSI and utilizing a tunable bandpass sigma-delta modulator in combination with digital signal processing techniques.
Sigma-delta ADCs use oversampling, noise shaping, digital filtering, and sample-rate decimation to obtain high dynamic range and high resolution. Because they use oversampling, the anti-alias filtering requirements of the ADC are reduced to allow simple, low-order designs. Theoretically, low-pass sigma-delta techniques can be extended to bandpass signal sampling (or RF/IF sampling) as typically required in a DSP-based digital radio implementation. Neglecting parasitic effects, a low-pass modulator can simply be frequency scaled to a bandpass frequency as with any filter design. However, the order of the modulator filter would have to approximately double to obtain the same noise shaping performance as the low-pass modulator. This makes the modulator much more difficult to design and to keep stable.
Fixed-tuned bandpass sigma-delta conversion has the potential for eliminating or simplifying the IF anti-alias filter requirements in a multistage RF-to-IF translator design for a digital receiver implementation. In fact, fixed-tuned bandpass modulators have been discussed in the engineering literature. Many fixed-tuned low-pass sigma-delta ADCs are commercially available today in integrated circuit form. In fact, this type of ADC is dominant in high performance consumer audio frequency applications.
However, a fixed-tuned design does not eliminate the frequency synthesizer and translator requirements in a radio design as would be required for a very large scale integration implementation. Further, due to the instability of high-order bandpass filter as required in an RF/IF design, a fixed-tuned sigma-delta converter would be too difficult and costly to implement. Thus, there lies a need for a tunable bandpass sigma-delta digital receiver.